Potential Energy

[MedPR]

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I was reading through some wikipremed flash cards and I got confused about energy storage in relation to like and unlike charges.

http://www.wikipremed.com/01physicscards.php?card=141


"The system of masses attracting one another by gravitational force is similar to two opposite charges attracting one another by electrostatic force. At infinite separation potential energy is zero."

Sure, since PEg = -GmM/r. If r is huge, then PEg is 0.
But then it says...

"Gravitational masses and unlike charges store potential energy as they are separated."

Isn't that the opposite of what is said above?

 

[MT Headed]

The zero of potential energy is an arbitrary decision. It's often easiest, mathematically, to define the zero to be infinite distance, and less-than-infinite to be less than that (i.e. negative).

So if two objects have a potential energy of -3 J, and then you separate them further so now they have a potential energy of -1 J, then you have 'stored' +2 J.

 

[MedPR]

The zero of potential energy is an arbitrary decision. It's often easiest, mathematically, to define the zero to be infinite distance, and less-than-infinite to be less than that (i.e. negative).

So if two objects have a potential energy of -3 J, and then you separate them further so now they have a potential energy of -1 J, then you have 'stored' +2 J.

So you're saying that as two unlike charges approach each other, they are using potential energy? I thought you need to input potential energy to oppose a force.. Like you input potential energy to lift something against gravity, or move a charged particle against its normal direction in an electric field. If two objects come together as a result of opposite charges, they aren't doing so at the expense of potential energy.

If you push them apart, against the attractive force, how can you be "storing" potential energy if you are actually using potential energy to initiate the separation?

 

[Ssina]

So you're saying that as two unlike charges approach each other, they are using potential energy? I thought you need to input potential energy to oppose a force.. Like you input potential energy to lift something against gravity, or move a charged particle against its normal direction in an electric field. If two objects come together as a result of opposite charges, they aren't doing so at the expense of potential energy.

If you push them apart, against the attractive force, how can you be "storing" potential energy if you are actually using potential energy to initiate the separation?

your actually applying work to separate two opposing charges, which is stored as PE.... it is like a box which you lift up against gravity and that energy is gained in PE, the box has the tendency to fall back to the ground to release it's PE... so the two opposing charges wants to get closer and release the stored PE (that they got due to separation).

this is opposite for similar charges, where they gain PE as you push them closer together, since they want to be apart.

 

[MedPR]

your actually applying work to separate two opposing charges, which is stored as PE.... it is like a box which you lift up against gravity and that energy is gained in PE, the box has the tendency to fall back to the ground to release it's PE... so the two opposing charges wants to get closer and release the stored PE (that they got due to separation).

this is opposite for similar charges, where they gain PE as you push them closer together, since they want to be apart.

I see. I think I understand now and I think I was confusing energy done/being done on the system.

When like charges are pushed together, the surroundings put energy into the system and that energy (following the law of conservation of energy) is then stored in the system as PE.

! thanks both of you.